feat(tui): dock approval gate + two-column diff viewer

Replace the floating approval modal (which blanked the whole UI) with an
opencode-style band that takes the input bar's slot while a gate is
pending: tool/tier/risk header above a side-by-side old|new diff, with
the session output and event stream still visible above it. The band
height adapts to the diff length.

Add a unified-diff parser that aligns removals/additions into split rows
(blank left cell for a create, blank right for a delete) and a
two-column renderer shared by the band preview and the ^x fullscreen
view. Wire plain a/r to approve/reject to match the advertised keys.

apps/tui-go/internal/app/view.go

@@ -22,7 +22,14 @@ func (m Model) View() string {
 		return m.theme.Screen.Render("correx — terminal too small")
 	}
 
-	mainH := m.height - statusH - footerH - inputH
+	bottom := m.renderInput()
+	bottomH := inputH
+	if m.displayState() == StateApproval {
+		bottomH = m.approvalBandHeight()
+		bottom = m.renderApprovalBand(bottomH)
+	}
+
+	mainH := m.height - statusH - footerH - bottomH
 	if mainH < 3 {
 		mainH = 3
 	}
@@ -30,16 +37,13 @@ func (m Model) View() string {
 	base := lipgloss.JoinVertical(lipgloss.Left,
 		m.renderStatus(),
 		m.renderMain(mainH),
-		m.renderInput(),
+		bottom,
 		m.renderFooter(),
 	)
 
 	if m.overlay != OverlayNone {
 		return m.renderOverlay(base)
 	}
-	if m.displayState() == StateApproval {
-		return m.renderApproval(base)
-	}
 	return base
 }